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First Report of Bacterial Spot of Peony Caused by a Xanthomonas sp. in the United States

In early May 2008 and 2009, peony samples (Paeonia spp.) with symptoms of leaf spot and blight were submitted to the Virginia Tech Plant Disease Clinic. The 2008 peony was an unknown cultivar from a northern Virginia landscape. The three cultivars (Dr. Alexander Fleming, Felix Crousse, and Karl Rosenfield) submitted in 2009 were from a commercial nursery in southwestern Virginia that was reporting leaf spot progressing to severe blight, which rendered plants unsalable, on 75% of a 1,219 m² block during a 10-day period of heavy rainfall. Bacterial streaming from spots was observed. On the basis of phenotypic and biochemical tests, the isolates were determined to be xanthomonads. Two isolates (one recovered from the 2008 sample and one from the 2009 sample) were used in the following work. Isolates were characterized by multilocus sequencing (MLST) (4). PCR reactions were prepared and cycled using 2X ImmoMix (Bioline, Tauton, MA) according to manufacturer's recommendations with an annealing temperature of 58°C. Template DNA was added by touching a single colony with a 20-μl pipette tip and placing the tip into the reaction mix for 1 min. Four bands of the expected size were visualized on an electrophoresis gel and cleaned products were sequenced in forward and reverse directions at the University of Chicago, Cancer Research Center DNA Sequencing Facility. Corresponding gene fragments of each isolate were identical. A consensus sequence (PAMDB Isolate ID No. 936) for each of the four gene fragments was constructed and compared with sequences in NCBI (http://www.ncbi.nlm.nih.gov/nuccore/) and PAMDB (http://genome.ppws.vt.edu/cgi-bin/MLST/home.pl) (1) databases using Blastn (2). No perfect match was found. Genetic distances between the peony isolates and all strains in PAMDB were determined by MegAlign (Lasergene; DNAStar, Madison, WI). The Xanthomonas strain most similar to the isolates recovered from the peony samples was Xanthomonas hortorum pv. hederae ICMP 1661 with a genetic distance of 0.023; this strongly suggests that the peony isolates belong to X. hortorum. For Koch's postulates, six surface-disinfested young leaflets from Paeonia lactiflora ‘Karl Rosenfield’ were inoculated by forcefully spraying a phosphate-buffered saline suspension of each bacterial isolate (~4.3 × 10⁹ CFU/ml) into the underside of the leaf until leaf tissue appeared water soaked. Controls were inoculated similarly with phosphate-buffered saline solution. Moist chambers with inoculated leaves were incubated at ambient temperature under two 48W fluorescent grow lights with 12 h of light and dark. Circular spots were observed on leaves inoculated with the 2009 and 2008 isolates in 18 and 20 days, respectively. No symptoms were observed on controls. Bacterial streaming from leaf spots was observed by phase-contrast microscopy; bacteria were isolated and confirmed to be identical to the original isolates by the methods described above. To our knowledge, this is the first report of a Xanthomonas sp. causing leaf spot and blight on peony. Although bacterial blight of peony has been attributed to a xanthomonad in recent years, the pathogen had not been further characterized (3).

References: (1) N. F. Almeida et al. Phytopathology 100:208, 2010. (2) D. J. Altschul et al. J. Mol. Biol. 215:403, 1990. (3) M. L. Gleason et al. Diseases of Herbaceous Perennials. The American Phytopathological Society, St. Paul, MN. 2009. (4) J. M. Young et al. Syst. Appl. Microbiol. 31:366, 2008.